Field of the Disclosure
Aspects of the present disclosure generally relate to hydrocarbon production using artificial lift and estimating efficiency and controlling the operation of a downhole pump disposed in a wellbore. More particularly, techniques of the present disclosure generally relate to improved diagnostics of downhole dynamometer data for control and troubleshooting of reciprocating rod lift systems.
Description of the Related Art
To obtain production fluids (e.g., hydrocarbons, oil, water, gas, and mixtures thereof), a wellbore is drilled into the earth to intersect a productive formation. Upon reaching the productive formation, pumps can be used in wells to help bring production fluids from the productive formation to a wellhead located at the surface. This is often referred to as providing artificial lift, as the reservoir pressure may be insufficient for the production fluid to reach the surface on its own (i.e., natural lift).
The production of fluids with a sucker-rod pump is common practice in the oil and gas industry. An oil well generally comprises a casing, a string of smaller steel pipe inside the casing and generally known as the tubing, a pump at the bottom of the well, and a string of steel rod elements, commonly referred to as sucker rods, within the tubing and extending down into the pump for operating the pump. Various devices as are well known in the art are provided at the top of the well for reciprocating the sucker rod to operate the pump.
It is desired to know the quantity of fluid entering the pump on each stroke (the pump “fillage”) for a number of purposes including, for example, to stop the pumping system periodically to allow more fluid to enter the wellbore or to control the speed of the pumping system so that it does not pump more fluid than enters the wellbore. Knowing the pump fillage also allows the total amount of fluid produced by the well to be calculated.
Other methods have previously relied on the shape of the graphical representation of the downhole card to compute the pump fillage. For example, U.S. Pat. No. 5,252,031 to Gibbs, entitled “Monitoring and Pump-Off Control with Downhole Pump Cards,” teaches a method for monitoring a rod pumped well to detect various pump problems by utilizing measurements made at the surface to generate a downhole pump card.
A surface dynamometer card is the plot of the measured rod loads at the various positions throughout a complete stroke. Each well can have a unique signature for its surface data. A dynamometer survey measures the load forces acting on a rod string during a complete pumping cycle (e.g., a downstroke and an upstroke) and records the forces on a chart or computer display. This display is often called a dynamometer card. The dynamometer card records changes in the rod load versus rod displacement, or changes in the rod load versus pumping time. During a pumping cycle, forces acting on the rod string cause changes in the rod load. Measurements of these rod loads reflect the operation of the subsurface pump and the surface unit. As evident from the graphical representation of the surface data, it may be difficult and/or inefficient to predict the shape, orientation, and span of the surface card.
For example, in a theoretical downhole, the bottom line may represent the plunger stroke and the top line may represent the sucker rod stroke. The bottom left point of the card may correspond to the start of the pump cycle with the standing valve and traveling valve closed. The top left point of the card may correspond to the opening of the standing valve during the upstroke. The top right point of the card may correspond to the closing of the standing valve at the top of stroke (TOS). The bottom right point of the card may correspond to the opening of the traveling valve during the downstroke.
A downhole dynamometer card (e.g., referred to as a pump card) is a plot of the calculated loads at various positions of pump stroke and represents the fluid load the pump applies to the bottom of the rod string. Measured surface data is used to calculate downhole data by solving the one dimensional damped wave equation. The wave equation model uses an iterated downhole friction factor. Friction continuously and irreversibly removed energy from the system. While in the case of the downhole data, the shape, orientation, and span may be more predictable than for the surface card, mechanical friction, fluid friction, and/or coulomb friction (referred to herein simply as “friction”) may cause errors or inaccuracies in the computing of the downhole card and should be properly handled in order to control the well efficiently.
The graphically represented downhole pump card may then be used to detect the various pump problems and control the pumping unit. Using downhole data, downhole conditions may be diagnosed such as, for example, pump off, gas interference, upstroke pump wear, and friction etc. In addition, other quantities such as pump fillage, fluid load, valve opening and closing, and net stroke, for example, may be deduced.
Owing to the diversity of card shapes, however, it can be difficult to make a diagnosis of downhole conditions solely on the basis of the shape of the graphical representation. Furthermore, in some instances, such graphical techniques may lead to inaccurate determinations of the pump fillage such that fluid production calculated therefrom may be incorrect. Also, given that a single field engineer may be responsible for thousands of wells at a time, properly diagnosing and controlling each well can be difficult if a visual analysis is required.
Accordingly, techniques and systems that rely less on human interpretation in determining the pump fillage are desirable.